The production of ethyl alcohol in dry-mill ethanol plants utilizing whole grains as feedstock is a major component of fuel alcohol in the U.S. A plant utilizing corn as feedstock can typically produce 2.7-2.8 gallons of ethanol and 17-18 lbs. of distillers grains with solubles (dry basis) per bushel of corn. While ethanol is the core product and the reason the processing plant exists, the byproduct, distillers grains with solubles, is also an important and significant revenue stream for an ethanol plant. Beyond increasing or improving the efficiency of ethanol production and yield from corn, optimizing the value and dollar return of the byproduct stream is critically important to maximize profitability.
One proposed method of increasing the value of the byproduct is to remove the crude oil contained within it for use in other industrial processes such as biodiesel production. Each bushel of corn that may produce 2.7 gallons of ethanol, also contains approximately 2 lbs of corn oil. Further, a typical 100 million gallon per year corn-to-ethanol production plant will discard approximately 40,000 short tons of crude corn oil per year in the distillers grains byproduct.
The total corn-to-ethanol industry in the United States is expected to grow to about 15 billion gallons per year by 2015 with an estimated 6 million short tons of corn oil in the byproduct. The byproduct, distillers grains with solubles, is a good livestock feed, especially for ruminants, and the oil content has feed value. However, as the amount of distillers grains increases and the supply of corn available for feeding decreases, it becomes economical to replace increasing amounts of corn in a ration with distillers grains.
When distillers grains are used as feed, at higher levels of inclusion of the byproduct the oil content in the byproduct begins to have deleterious effects on the animal, such as reduced milk fat production in dairy cows, reduced conception rates, soft fat in pork and bacon due to a high level of unsaturation, as well as reduced feed intake and weight gain in beef feedlot cattle. Therefore, there are significant advantages to removing the oil from the byproduct, such as increasing the level of effective inclusion in livestock diets and allowing the oil to be directed toward higher value industrial processes or feed markets.
Solutions have been attempted to remove oil from grains. For example, soybeans, canola, sunflowers, cottonseed, peanuts, and other commodities are valued for their oils. Technologies, such as solvent extraction or extrusion, exist for the efficient and economical removal of oil from these commodities. However, these same technologies are generally applied to corn at the front end of an ethanol production process, that is, prior to distillation. Unfortunately, the oil content in corn grain is typically only between 3.5% and 4.0% and removing it from the grain is not very cost effective. An alternative solution is to fractionate the germ from the rest of the corn kernel for oil removal, as the germ contains approximately 25% oil. It is by this fractionation method that commercial corn oil is typically obtained in the wet milling industry. Unfortunately, in dry milling, corn components such as germ, pericarp and endosperm do not separate easily or cleanly, as compared to wet milling processes. For example, the germ can be separated by a dry milling process, however it is at the expense of some starch loss which results in lower ethanol plant productivity and profit.
In addition to the current issues in removing oil, in the ethanol production process, thin stillage typically can only be condensed to a total solids content ranging between 20% and 30%. Further, the gums and waxes in the thin stillage cause the solubles to become very viscous when it is condensed and these components often cause fouling of condensers. Further, while it is possible to use a high speed centrifuge to remove crude corn oil from thin stillage, the industry has only been able to achieve between 25%-80% removal of total oil in thin stillage due to the presence of these gums and waxes which are bound with the oil and are loathe to relinquish their bonds. At best, solutions to remove or separate crude oil in thin stillage necessitate the use of very high speed, costly centrifuges to recover a portion of the oil as the gums within stillage bind the oil. Moreover, the 25%-80% recovery of total oil in thin stillage is equal to only approximately 48% of the total oil available in whole stillage (60% of the oil from whole stillage present in thin stillage multiplied by 80% recovery).
Separation of oil from the grains, which constitutes a majority of the oil, cannot typically be accomplished by centrifugation because the oil is still bound within the germ. Therefore, solutions often include extraction methods. Typical extraction methods for oilseeds include solvent extraction with organic solvents such as hexane, benzene, ethanol, methanol and others, as well as extruding techniques which apply very high pressure and temperature to the material to ‘squeeze’ the oil out. Unfortunately, these methods are high cost compared with the relatively low amount of oil contained within the distillers grains (about 7.5% on a dry basis) and often include toxic chemicals. In addition, solvent extraction is not oil-specific but also extracts other components soluble in organic solvents, such as the gums which are also present in thin stillage, resulting in a crude oil with a high level of impurities requiring further refining.
Accordingly, there is a need in the art to more efficiently reduce or extract the oil content of distillers grains byproduct in a dry mill ethanol plant. There is also a need to increase the efficiency and yield of crude oil removal from distillers whole stillage or thin stillage while increasing the overall profitability of ethanol production plants by improving the value of the byproducts.